This setup, appart of the manual sync, had some issues to be addressed:

Because of packaging activities /var was always nearly full, so it had to be increased

Because of doubling the memory some months ago, there was less swap than current RAM size (2G), also something to fix

The hardware choice wasn't very difficult, as I tend to like Seagate, so balancing price, capacity and availability decided for a couple of ST3500418AS. These are SATA-II, while my motherboard is SATA-I only but aren't they supposed to be backwards compatible? Well, they are, but you have to setup a jumper to lower interface speed, otherwise the disc isn't even recognized by the motherboard.

Buying the discs had some more difficulties. First tried Alternate, but this time they pretend me to pay the SGAE[es] tax for media (which is around 12 euros per disc), despite I clearly explained these were system discs to be mounted in RAID (and the tax is supposed only to apply non-system drives). Phoned them even, but no way, so I finally rejected the discs and went Optize, which doesn't seem to have the supposedly legal problem Alternate has with declaring system discs. They were served on time and for less than 90 euros, so bravo for them :).

After having the bare metal, initially these options for migration were considered:

Buy a 2.5 disc, copy current data (a 250 Gb disc is enough), install the new system, copy back

Buy a hard disc enclosure for the remaining good disc, install the new system, use the enclosure to copy data back

But in the end I got it with a fourth option based on this later one: install a new system with all the RAID setup, disconnect second drive (like if the array had failed), reconnect and copy contents of old drive to new system, restore second RAID drive and add it again to the array, so it gets synced again. Nothing to buy and more fun to see how fast the MD rebuilds the array.